I have a 454 mag, that I had some head work done after I dropped a valve. Before the valve went, and was running at 48-5000 rpm. After the head work I can only get about 4200 out of it. When the valve dropped, the head gasket went, and the head had to be shaved to take out the defects. I was told this increased my compression. Do I just need to reprop after the changes to the motor?

Added compression does not lose power.

Was something else changed ? Valve springs, Cam, carb, intake, or etc ?

If 100% no other changes, possible springs should have been replaced and/or you have a 'put back together issue' like vaccum leak, incorrect adjusted rocker arms, throttle cable not opening carb all the way, ignition timing is off, etc,etc

He changed all the valve springs from what I paid for anyway, but was told he put the exact same ones only new back in. It does have an a/m Edelbrock cam and intake, but that was there before the work was done. Possible everything just need to wear in a bit? Thanks for the insight.

800rpm after 'things wear in' ? No way Jose.

Somethings off big time.
Throttle blades not opning fully
timing way off
compression off
someone put a 3-4" higher pitch prop on without you looking ?
something, something, something. Oh, something pretty big.

SB is dead on .

Start with checking your plug wires. Its a very common mistake and easy to do . It would account for 800 RPM

Then check your
prop
plugs
carb linkage
compression test
Timing

One of these will tell you what is wrong

Good Luck

Thanks for the tips guys, Ill get to checking it all out.

I had a an experience with timing that picked up a lot of RPM when corrected.

A buddy of mine had an experience with his mechanic tightening the rockers too tight that caused a large loss of RPM.

These are two possibilities.

Did you get a cylinder compression check? +125 psi on all?
If not, check your rockers adjustment and if right, pistons and rings???
If yes, then your ignition timing is too retarded, should be over 32 degre advance @ 3200rpm.
If timing is right, check for a vacuum leak around the intake. (I use a non-lit propane torch, just shoot the gas into the carb to hear and reconize the different rpm and then go around the carb and intake, it works well).
if everything up there is right then open your carb (probably a holley) and check your power valve diaphram. A backfire usually destroy them.
That my 2 cents worth.
Josh

Thanks for all the info guys. Hopefully one of the above suggestions will take care of the problem.